The design of a high performance receiver is made challenging by various design constraints. First, high performance is required for many applications. High performance can be described by the linearity of the active devices (e.g. amplifiers, mixers, etc.) and the noise figure of the receiver. Second, for some applications such as in a cellular communication system, power consumption is an important consideration because of the portable nature of the receiver. Generally, high performance and high efficiency are conflicting design considerations.
An active device has the following transfer function:Y(x)=a1·x+a2·x2+a3·x3+higher order terms,  (1)where x is the input signal, y(x) is the output signal, and a1, a2 and a3 are coefficients which define the linearity of the active device. For simplicity, higher order terms (e.g. terms above third order) are ignored. For an ideal linear, active device, the coefficients a2 and a3 are 0.0 and the output signal is simply the input signal scaled by a1. However, all active devices experience some amount of non-linearity which is quantified by the coefficients a2 and a3. Coefficient a2 defines the amount of second order non-linearity and coefficient a3 defines the amount of third order non-linearity.
Most communication systems are narrow band systems which operate on an input RF signal having a predetermined bandwidth and center frequency. The input RF signal typically comprises other spurious signals located throughout the frequency spectrum. Non-linearity within the active device causes intermodulation of spurious signals, resulting in products which may fall into the signal band.
The effect of second order non-linearity (e.g. those caused by the x2 term) may be reduced or eliminated by careful design methodology. Second order non-linearity produces products at the sum and difference frequencies. Typically, the spurious signals which can produce in-band second-order products are located far away from the signal band. However, third order non-linearity are more problematic. For third order non-linearity, spurious signals x=g1·cos(w1t)+g2·cos(w2t) produce products at the frequencies (2w1−w2) and (2w2−w1). Thus, near band spurious signals (which are difficult to filter) can produce third order intermodulation products falling in-band, causing degradation in the received signal. To compound the problem, the amplitude of the third-order products are scaled by g1·g22 and g12g2. Every 1 dB increase in the input RF signal results in 1 dB increase in the output RF signal but 3 dB increase in the third order products.
The linearity of a receiver (or the active device) can be characterized by the input-referred third-order intercept point (IIP3). Typically, the output RF signal and the third-order intermodulation products are plotted versus the input RF signal. As the input RF signal is increased, the IIP3 is a theoretical point where the desired output RF signal and the third-order products become equal in amplitude. The IIP3 is an extrapolated value since the active device goes into compression before the IIP3 point is reached.
Receivers are employed for many communication applications, such as cellular communication systems. Example cellular communication systems include Code Division Multiple Access (CDMA) communications systems, Time Division Multiple Access (TDMA) communication systems, and analog FM communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present disclosure and incorporated by reference herein.
In cellular applications, it is common to have more than one communication system operating within the same geographic coverage area. Furthermore, these systems can operate at or near the same frequency band. When this occurs, the transmission from one system can cause degradation in the received signal of another system. CDMA is a spread spectrum communication system which spreads the transmit power to each user over the entire 1.2288 MHz signal bandwidth. The spectral response of an FM-based transmission can be more concentrated at the center frequency. Therefore, FM-based transmission can cause jammers to appear within the allocated CDMA band and very close to the received CDMA signal. Furthermore, the amplitude of the jammers can be many times greater than that of the CDMA signal. These jammers can cause third-order intermodulation products which can degrade the performance of the CDMA system.
Typically, to minimize degradation due to intermodulation products caused by jammers, the receiver is designed to have high IIP3. However, design of a high IIP3 receiver requires the active devices within the receiver to be biased with high DC current, thereby consuming large amounts of power. This design approach is especially undesirable for cellular application wherein the receiver is a portable unit powered by a battery and power is limited.
Several techniques have been deployed in the prior art to address the need for high IIP3. One such technique, which also attempts to minimize power consumption, is to implement the gain stage with a plurality of amplifiers connected in parallel and to selectively enable the amplifiers as higher IIP3 is needed. This technique is disclosed in detail in U.S. Pat. No. 6,069,525, entitled “DUAL MODE AMPLIFIER WITH HIGH EFFICIENCY AND HIGH LINEARITY”, assigned to the assignee of the present disclosure and incorporated by reference herein. Another technique is to measure the received RF (radio-frequency) signal power and adjust the gain of the amplifiers based on the amplitude of the RF signal power. This technique is disclosed in detail in U.S. Pat. No. 5,722,061, entitled “METHOD AND APPARATUS FOR INCREASING RECEIVER POWER IMMUNITY TO INTERFERENCE”, assigned to the assignee of the present disclosure and incorporated by reference herein. These techniques improve the IIP3 performance.
An example block diagram of a receiver architecture of the prior devices is shown in U.S. Pat. No. 6,498,926 entitled “PROGRAMMABLE LINEAR RECEIVER HAVING A VARIABLE IIP3 POINT”, which is assigned to the assignee of the present disclosure and the patent is incorporated by reference in its entirety as if set out in full herein.
Receiver architectures to the present date have several drawbacks. First, the active devices are typically biased to a high DC current to provide the highest required IIP3. This has the effect of operating the receiver at the high IIP3 operating point at all times, even though high IIP3 is not required most of the time. Second, the high IIP3 can be improved by adjusting the gain of Automatic Gain Control (AGC) amplifier; however, lowering the gain of the amplifier can degrade the noise figure of the receiver.